1. Field of the Invention
The present invention relates to an aluminum alloy with high seawater corrosion resistance and relates to a plate-fin heat exchanger including a heat transfer portion with seawater as a coolant, the heat transfer portion being composed of the aluminum alloy. Hereinafter, aluminum is also referred to as “Al”.
2. Description of the Related Art
Aluminum alloys have high specific strength and high thermal conductivity and thus have been widely used as materials for compact lightweight heat exchangers. Typical examples of heat exchangers composed of aluminum alloys include fin-and-tube heat exchangers for use in household air conditioners and automotive radiators. Industrial plate-fin heat exchangers are composed of titanium and utilize seawater as coolants. Attempts have been made to produce plate-fin heat exchangers composed of more economical aluminum alloys.
Such plate-fin heat exchangers including heat transfer portions with seawater as coolants are exposed to stringent corrosive environments because of use in seawater environments. Thus, titanium, which has excellent corrosion resistance, is currently used. Aluminum alloys have high corrosion resistance. However, when aluminum alloys are used for such plate-fin heat exchangers in place of titanium, sufficient corrosion protection is required.
In general, examples of corrosion protection of aluminum alloys constituting such plate-fin heat exchangers include formation of anodic oxidation coatings, electrolytic protection, and formation of coatings with paint. In the case where corrosion protection is applied to heat exchangers, measures to incorporate an inhibitor into a coolant are also utilized.
However, plate-fin heat exchangers are of a single pass type. That is, a coolant passes through an exchanger and is then drained out of the system. The coolant does not circulate. Thus, the corrosion protection utilizing an inhibitor is not appropriate. From the viewpoint of economy, the corrosion protection by formation of a coating is suitable.
Examples of coatings usable for aluminum alloys constituting heat exchangers include various types of inorganic, organic, and organic-inorganic hybrid coatings. These coatings are practically used. Methods of forming coatings for heat exchangers are described in, for example, Japanese Unexamined Patent Application Publication No. 2003-88748 (Patent Document 1) and Japanese Unexamined Patent Application Publication No. 2004-42482 (Patent Document 2).
Patent Document 1 discloses the formation of a polyaniline coating for an aluminum alloy not constituting a target plate-fin heat exchanger with seawater but constituting a fin-and-tube heat exchanger for use in a household air conditioner or an automotive radiator.
Patent Document 2 discloses that a coating is formed on a composite underlying coating including a boehmite treatment coating or a silicate treatment coating for an aluminum alloy constituting a fin-and-tube heat exchanger for use in a household air conditioner or an automotive radiator as in Patent Document 1 to improve adhesion.
Non-Patent Document 1 (Akihiro YABUKI, Hiroyoshi YAMAGAMI, Takeshi OWAKI, Kiyomi ADACHI, and Koji NOISSHIKI. “Self-Repairing Property of Anticorrosive Coating for Aluminum Alloy”, Conference Proceedings of Material and Environment, 3-4 (2004)) discloses that an anticorrosive trifluororesin coating for a single-pass heat exchanger has self-repairing properties.
Japanese Unexamined Patent Application Publication No. 2006-169561 (Patent Document 3) discloses, as the improvement of the anticorrosive trifluororesin coating, a self-repairing anticorrosive coating for an aluminum alloy, the coating being composed of a trifluororesin containing 0.1 to 10 percent by volume of at least one element selected from zinc, titanium, manganese, aluminum, and niobium. In heat exchangers utilizing seawater as a cooling medium, surfaces of heat exchangers are fragile. This is a method of preventing the rapid expansion of a flaw due to severe corrosion by seawater if once the flaw is formed. That is, the anticorrosive trifluororesin coating containing the metal powder has self-repairing properties in which when the coating is flawed, the coating is repaired.